Subunit arrangement in the dodecameric chloroplast small heat shock protein Hsp21

Protein Sci. 2011 Feb;20(2):291-301. doi: 10.1002/pro.560. Epub 2010 Dec 23.


Unfolding proteins are prevented from irreversible aggregation by small heat shock proteins (sHsps) through interactions that depend on a dynamic equilibrium between sHsp subunits and sHsp oligomers. A chloroplast-localized sHsp, Hsp21, provides protection to client proteins to increase plant stress resistance. Structural information is lacking concerning the oligomeric conformation of this sHsp. We here present a structure model of Arabidopsis thaliana Hsp21, obtained by homology modeling, single-particle electron microscopy, and lysine-specific chemical crosslinking. The model shows that the Hsp21 subunits are arranged in two hexameric discs, similar to a cytosolic plant sHsp homolog that has been structurally determined after crystallization. However, the two hexameric discs of Hsp21 are rotated by 25° in relation to each other, suggesting a role for global dynamics in dodecamer function.

MeSH terms

  • Amino Acid Sequence
  • Arabidopsis / chemistry
  • Arabidopsis / metabolism
  • Arabidopsis Proteins / chemistry*
  • Arabidopsis Proteins / metabolism
  • Chloroplasts / chemistry*
  • Cross-Linking Reagents
  • Cytosol / chemistry
  • Cytosol / metabolism
  • Heat-Shock Proteins / chemistry*
  • Heat-Shock Proteins / metabolism
  • Image Processing, Computer-Assisted
  • Lysine / chemistry
  • Lysine / metabolism
  • Microscopy, Electron
  • Models, Molecular
  • Molecular Sequence Data
  • Protein Conformation
  • Protein Multimerization
  • Protein Subunits / chemistry
  • Protein Subunits / metabolism
  • Sequence Alignment
  • Structural Homology, Protein


  • Arabidopsis Proteins
  • Cross-Linking Reagents
  • HSP21 protein, Arabidopsis
  • Heat-Shock Proteins
  • Protein Subunits
  • Lysine